(a) Field of the Invention
The present invention relates generally to antennas and, more particularly, to low profile receiving/transmitting antennas, such as flat antennas, used in communication systems.
(b) Description of Related Art
Satellites are commonly used to relay or communicate electronic signals, including audio, video, data, audio-visual, etc. signals, to or from any portion of a large geographical area, such as the continental United States. A satellite-based signal distribution system generally includes an earth station that compiles one or more individual audio/visual/data signals into a narrowband or broadband signal, modulates a carrier frequency band with the compiled signal and then transmits (uplinks) the modulated signal to one or more, for example, geosynchronous satellites. The satellites amplify the received signal, shift the signal to a different carrier frequency band and transmit (downlink) the frequency shifted signal to earth for reception at individual receiving units. Likewise, individual receiving units may transmit a signal, via a satellite, to the base station or to other receiving units.
Many satellite communication systems, including some commercial and military mobile communication systems as well as a direct-to-home satellite system developed by DIRECTV.RTM. and known commercially as DSS.RTM., use millimeter wave (MMW) carrier frequencies, such as Ku band (ranging from approximately 12 GHz to 18 GHz), to transmit a signal from the satellite to one or more receiver units and/or vise-versa.
In particular, the DSS system uses an uplink signal having 16 right-hand circular polarized (RHCP) transponder signals and 16 left-hand circular polarized (LHCP) transponder signals modulated onto the frequency band between about 17.2 GHz and about 17.7 GHz. The satellites associated with the DSS system shift the uplink transponder signals to carrier frequencies ranging from approximately 12.2 GHz to approximately 12.7 GHz and transmit these frequency-shifted transponder signals back to earth for reception at each of a plurality of individual receiver units. At each individual receiver unit, a receiving antenna, typically comprising a parabolic dish antenna, is pointed in the general direction of the transmitting satellite (or other transmitting location) to receive the broadband signal.
While the dish antennas associated with the DSS and other communication systems are generally acceptable for receiving satellite signals at stationary receivers, these antennas are typically too large and cumbersome to mount on mobile receivers, such as cars, buses, trucks, tanks, airplanes, helicopters, etc. With mobile receivers, it is desirable to use a receiving/transmitting antenna which is small, to reduce the space necessary for mounting the antenna, and which has a low profile, to reduce the wind resistance caused by the antenna.
Furthermore, mobile antennas must be capable of receiving satellite signals at a range of angles of incidence due to the fact that the angle of incidence of any particular satellite signal changes as the mobile antenna moves across a large geographical area such as the continental United States. In fact, the angle of incidence of a typical geosynchronous satellite signal changes by approximately 23 degrees as an antenna travels across the continental United States.
In the past, flat mobile antennas have been developed using electronically steered phased array antennas which have a number of individual receiving elements positioned in a flat plane. The phase of each of the receiving elements is electronically controlled in a manner which steers the beam of the antenna to different azimuth and elevation angles without moving the antenna. While phased array antennas produce acceptable mobile receiving antennas, they are typically hard to maintain and are expensive to build due to the complexity of the electronic control associated with these antennas.
Continuous transverse stub (CTS) antennas may be used to produce receiving/transmitting antennas. However, it is difficult to design small, low profile CTS antennas that have the required efficiency or antenna gain that is necessary to receive satellite communication signals adequately due, in part, to the fact that CTS antennas do not collect radiation across the entire aperture associated with the antenna and due, in part, to the fact that CTS antennas tend to retransmit some of the collected radiation.